Remove Expired FlushQueueNoBeginMainFrame histogram.

It is obvious that even though the renderer has requested main frames
the browser may not deliver it until a much later time. So we will
definitely keep the raf fallback timer code.

BUG=975271

Change-Id: I868bc32a7e95c59585dfa72fe3c56959979a17f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1703092Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678268}

content/renderer/input/main_thread_event_queue.cc

@@ -477,8 +477,9 @@ static bool IsAsyncTouchMove(
 }
 
 void MainThreadEventQueue::RafFallbackTimerFired() {
-  UMA_HISTOGRAM_BOOLEAN("Event.MainThreadEventQueue.FlushQueueNoBeginMainFrame",
-                        true);
+  // This fallback fires when the browser doesn't produce main frames for a
+  // variety of reasons. (eg. Tab gets hidden). We definitely don't want input
+  // to stay forever in the queue.
   DispatchRafAlignedInput(base::TimeTicks::Now());
 }
 

tools/metrics/histograms/histograms.xml

@@ -37070,6 +37070,9 @@ uploading your change for review.
 
 <histogram name="Event.MainThreadEventQueue.FlushQueueNoBeginMainFrame"
     enum="BooleanHit" expires_after="M77">
+  <obsolete>
+    Expired 2019-07.
+  </obsolete>
   <owner>dtapuska@chromium.org</owner>
   <summary>
     Whether the Begin Main Frame was not received and the queue generated a